Pre-press color proofing is a procedure used by the printing industry for creating representative images of printed material without the high cost and time required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy customers requirements. By utilizing pre-press color proofing time and money can be saved.
One such commercially available image processing apparatus, disclosed in commonly assigned U.S. Pat. No. 5,268,708, describes image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying a sufficient amount of thermal energy to the dye donor material to form an intended image. This image processing apparatus is comprised of a material supply assembly or carousel; lathe bed scanning subsystem, which includes a lathe bed scanning frame, translation drive, translation stage member, and printhead; vacuum imaging drum; and thermal print media and dye donor material exit transports.
The operation of the image processing apparatus comprises metering a length of the thermal print media, in roll form, from the material assembly or carousel. The thermal print media is cut into sheets, transported to the vacuum imaging drum, registered, wrapped around, and secured onto the vacuum imaging drum. A length of dye donor material, in roll form, is metered out of the material supply assembly or carousel, and cut into sheets. The dye donor material is transported to and wrapped around the vacuum imaging drum, such that it is superposed in the registration with the thermal print media.
After the dye donor material is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine writes an image on the thermal print media as the thermal print media and the dye donor material on the spinning vacuum imaging drum is rotated past the printhead. The translation drive traverses the printhead and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the dye donor material is removed from the vacuum imaging drum without disturbing the thermal print media that is beneath it. The dye donor material is transported out of the image processing apparatus by the dye donor material exit transport. Additional sheets of dye donor material are sequentially superposed with the thermal print media on the vacuum imaging drum, and imaged onto the thermal print media as described above until the intended image is completed. The completed image on the thermal print media is unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport.
The vacuum imaging drum is cylindrical in shape and includes a hollowed-out interior portion. A plurality of holes extending through the drums permit a vacuum to be applied from the interior of the vacuum imaging drum for supporting and maintaining the position of the thermal print media and dye donor material as the vacuum imaging drum rotates.
Although the operation of prior art image processing apparatus is satisfactory, it is not without drawbacks. The donor and receiver media must be held tightly against the surface of the vacuum imaging drum to prevent irregular surface conditions caused by factors such as folds, creases, wrinkles, or trapped air. Such irregular surface conditions could adversely affect the imaging process, or cause the media to fly-off at high drum speeds causing damage to the image processing apparatus. To achieve a flat surface, considerable vacuum force is exerted. A solution that would decrease or eliminate folds, creases, wrinkles, or trapped air would be advantageous and would allow higher drum speeds which would increase throughput of the imaging apparatus.
To prevent folds, creases, wrinkles, or trapped air when the sheets are wrapped around a vacuum drum, conventional methods include using clamps as a supplement to vacuum. For example, clamps used with a vacuum imaging drum are disclosed in U.S. Pat. No. 5,159,352 (Ferla et al.) and U.S. Pat. No. 4,660,825 (Umezawa). However, such solutions are mechanically complex. Moreover, even slight protrusion of a clamp from the surface circumference of the drum is prohibitive at high speeds, for example, 600 RPM and higher, and measures must then be taken to prevent mechanical contact with the printhead.
Other approaches for stretching a sheet on a drum include those used with printing press paper transfer mechanisms. U.S. Pat. No. 4,852,488 (Abendroth et al.) discloses the use of sucker fingers inside a sheet transfer drum. These sucker fingers are disposed within slots on the surface of the drum. The fingers grab the sheet at discrete points, then provide stretching action as the fingers are moved diagonally within the slots. U.S. Pat. No. 5,186,107 (Wieland) discloses suction elements that perform a similar stretching function on a press transfer drum. The suction elements are at extreme ends of the sheet in diagonal slots so that the stretching action pulls the media outward from its center. Such solutions are mechanically complex, however, and would be likely to distort the media used in an image processing apparatus.
Mechanical clamps, fingers, slots, or other structures affect the weight distribution of drum components and any imbalance could easily cause the drum to go out of round when rotating at high speeds. There is a need for providing increased suction and stretching force for donor and receiver media loaded onto a vacuum imaging drum without increasing the drum vacuum or adding unnecessary additional mechanical components.